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We said that the smoking gun for human 
impact on climate 

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is the instrumental temperature record and 
the warming that it's shown since the 

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1970s, and that the models are able to 
explain that warming if you 

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tell them about rising greenhouse gas 

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concentrations, and they're not if you 
don't. 

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But that isn't really fair because it 
makes it seem like 

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the whole thing, our confidence in it, is 
based on the models. 

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And in fact, the reason why I personally 

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believe that the models know what they're 
talking about 

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comes from comparisons also with climate 
changes in 

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the deeper past, what are called 
paleoclimatology constraints. 

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if we look back further in the past, we 
have much larger changes in 

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Earth's climate to work with, but, 
unfortunately, 

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also, the uncertainties become bigger as 
well. 

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It's a trade off in that regard. 

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One of the most important findings 

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in paleoclimatology is the close link 
between atmospheric CO2 concentration

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and ice volume and temperature going back 
through time 

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through the ice ages as revealed in the 
ice cores. 

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This plot, we'll talk more about in the 
coming lectures, 

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but it demonstrates a very tight 
correlation between CO2 and temperature. 

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Makes it clear that there is a CO2 impact 
on Earth's temperature. 

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And in fact, what we'll see is that we can 
quantitatively boil this down and try to 

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derive a value of the climate sensitivity. 
Delta T(2x), the amount of warming 

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that we expect from doubling CO2. 

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And we get pretty much the same answer 
from looking at these CO2 

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and temperature fluctuations as we do from 

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looking at the recent instrumental 
temperature record. 

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It gives us a separate 
independent check on the role of CO2 in 

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Earth's climate. 
Reconstruction 

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of Earth's climate in the past is based on 
what are called 

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proxy measurements of various climate 
parameters, mostly temperature 

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but also other things like water 
availability or or winds 

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or things like that can be pulled out from 
proxy measurements. 

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It's a proxy because we don't have a 
thermometer 

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from thousands of years ago to actually 
measure the temperature, 

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so we have to come up with other clever 
things 

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that we can use to guess what the 
temperature was. 

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And we do the best we can at that. 

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It's certainly not as good as having a 
real thermometer, but, we can, we 

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can get a lot of mileage by looking at 
things that we'll talk about. 

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Like the widths of tree rings of old 
fossil logs from ancient trees can tell us 

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about temperature and drought. 
Bore hole temperatures are temperatures 

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down inside an ice sheet or a a big 
rock or something like 

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that we can use to back out changes 
in temperatures in the past. 

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Basically, in general, a lot of the Paleo 
climate records come from ice cores that 

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accumulated in Antarctica or Greenland or 
on the tops of mountains little mountain 

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glaciers, and also sediment cores from the 
ocean and from lakes. 

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Let's hear about some paleo climate 
proxies. 

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[MUSIC]